The invention relates to a signal lamp comprising a box-shaped housing having an open end, a number of LEDs being accommodated in the housing and the open end of the housing being closed by a spreading window.
Such signal lamps are known per se. They are used, inter alia, in signal lighting for controlling different types of traffic, such as in traffic lights. Lamps of this type comprise a large number of light-emitting diodes (LEDs), which are regularly distributed on the entire inner surface of the housing. The spreading window of such a signal lamp ensures a proper distribution of the light intensity and, if necessary, a homogeneous brightness distribution. It is noted that "distribution of the light intensity" is to be understood to mean in this context, the angle-dependent distribution of the light intensity. "Brightness distribution" is to be understood to mean in this context, the angle-independent light distribution on the surface of the spreading window of the signal lamp.
It is also known to structure the spreading window of a signal lamp with LEDs in such a manner that each of the LEDs is provided with an optical system of its own which is integrated in the spreading window. By virtue of the presence of such an optical system, the brightness distribution of the window is optimal during operation of the lamp. The currently used signal lamps comprise more than 400 LEDs. However, there is a tendency to reduce this number. This tendency is also caused by the fact that LEDs having a higher light output are becoming available. For example, the latest signal lamps only comprise 150-200 LEDs.
Signal lamps of the above-mentioned type have an important drawback. It has been found that failure of one or more of the LEDs of such a lamp gives rise to an inhomogeneous brightness distribution on the surface of the spreading window. This disadvantage manifests itself in the form of dark spots on the window of the lamp. As a result, after failure of one or more LEDs, the known signal lamps no longer meet the requirements as regards the homogeneity of the brightness distribution. This problem increases as the number of LEDs per lamp decreases.